The present invention relates in general to the art of medical diagnostic imaging and in particular to imaging blood flow using magneto-motive optical Doppler tomography (MM-ODT), Optical Coherence Tomography, or Ultrasound, which combines an externally applied temporally oscillating high-strength magnetic field with ODT, OCT, or Ultrasound to detect erythrocytes moving according to the field gradient.
The accurate determination of location and flow velocity of moving particles in highly scattering media, such as blood flow, is important for medical diagnostics. While the measurements of blood flow in the coronary arteries is an important aspect in diagnosing coronary artery diseases. Numerous non-invasive approaches have been developed using techniques such as Doppler ultrasound, conventional angiography, laser Doppler flowmetry and magnetic resonance angiography.
One common sensing technique involves the use of ultrasound. Using this technique, ultrasound is directed into the body of the patient and tiny particles such as red blood cells, which are suspended in the blood plasma, scatter the ultrasonic energy back towards the receiver or transducer. The transducer then converts the back-scattered ultrasonic energy into an electrical signal that is processed in some known manner to determine the presence of a flow and an estimate of the flow velocity.
Magnetic resonance imaging (MRI) is based on an imaging technique for magnetically exciting nuclear spins in a subject positioned in a static magnetic field by applying a radio-frequency (RF) signal of the Larmor frequency, and reconstructing an image using MR signals induced by the excitation. MRI is widely applied in clinical medicine because of its capability of clearly depicting the slightest tissue of human brain in vivo.
Magnetic resonance angiography (MRA) provides detailed angiographic images of the body in a non-invasive manner. In conventional MRA, which does not use contrast agents, magnetic resonance signal from flowing blood is optimized, while signal from stationary blood or tissue structures is suppressed. In contrast-enhanced MRA, a contrast agent is injected into the blood stream to achieve contrast between flowing blood and stationary tissue.
The commonly known echo planar imaging (EPI) is a rapid MRI technique, which is used to produce tomographic images at high acquisition rates, typically several images per second. Functional magnetic resonance imaging (fMRI) has been found useful in perfusion and/or diffusion studies and in dynamic-contrast studies, etc. However, images obtained in EPI experiments tend to be vulnerable to an artifact known as “ghosting” or “ghost images.”
Optical coherence tomography (OCT) is a technology that allows for non-invasive, cross-sectional optical imaging of biological media with high spatial resolution and high sensitivity. OCT is an extension of low-coherence or white-light interferometry, in which a low temporal coherence light source is utilized to obtain precise localization of reflections internal to a probed structure along an optic axis. This technique is extended to enable scanning of the probe beam in the direction perpendicular to the optic axis, building up a two-dimensional reflectivity data set, used to create a cross-sectional gray-scale or false-color image of internal tissue backscatter.
OCT uses the short temporal coherence properties of broadband light to extract structural information from heterogeneous samples such as biologic tissue. OCT has been applied to imaging of biological tissue in vitro and in vivo. Systems and methods for substantially increasing the resolution of OCT and for increasing the information content of OCT images through coherent signal processing of the OCT interferogram data have been developed to provide cellular resolution (i.e., in the order of 5 micrometers). During the past decade, numerous advancements in OCT have been reported including real-time imaging speeds.
In diagnostic procedures utilizing OCT, it would also be desirable to monitor the flow of blood and/or other fluids, for example, to detect peripheral blood perfusion, to measure patency in small vessels, and to evaluate tissue necrosis. Another significant application would be in retinal perfusion analysis. Accordingly, it would be advantageous to combine Doppler flow monitoring with the above micron-scale resolution OCT imaging in tissue.
Conventional OCT imaging primarily utilizes a single backscattering feature to display intensity images. Functional OCT techniques process the backscattered light to provide additional information on birefringence, and flow properties. (See for example, Kemp N J, Park J, Zaatar H N, Rylander H G, Milner T F, High-sensitivity determination of birefringence in turbid media with enhanced polarization-sensitive optical coherence tomography, Journal of the Optical Society of America A: Optics Image Science and Vision 2005, 22(3):552-560; Dave D P, Akkin T, Milner T E, Polarization-maintaining fiber-based optical low-coherence reflectometer for characterization and ranging of birefringence, Optics Letters 2003, 28(19): 1775-1777; Rylander C G, Dave D P, Akkin T, Milner T E, Diller K R, Welch M, Quantitative phase-contrast imaging of cells with phase-sensitive optical coherence microscopy, Optics Letters 2004, 29(13):1509-1511; de Boer J F, Milner T E, Ducros M G, Srinivas S M, Nelson J S, Polarization-sensitive optical coherence tomography, Handbook of Optical Coherence Tomography, New York: Marcel Dekker, Inc., 2002, pp 237-274.)
Since the ability to characterize fluid flow velocity using OCT was demonstrated by Wang et al., several phase resolved, real-time optical Doppler tomography (ODT) approaches have been reported. (See for example, Chen Z P, Milner T E, Dave D, Nelson J S, Optical Doppler tomographic imaging of fluid flow velocity in highly scattering media, Optics Letters 1997, 22(1):64-66; Wang X J, Milner T E, Nelson J S.
Optical Doppler tomography (ODT) combines Doppler velocimetry with optical coherence tomography (OCT) for noninvasive location and measurement of particle flow velocity in highly scattering media with micrometer-scale spatial resolution. The principle employed in ODT is very similar to that used in radar, sonar and medical ultrasound. ODT uses a low coherence or broadband light source and optical interferometer to obtain high spatial resolution gating with a high speed scanning device such as a conventional rapid scanning optical delay line (RSOD) to perform fast ranging of microstructure and particle motion detection in biological tissues or other turbid media.
To detect the Doppler frequency shift signal induced by the moving particles, several algorithms and hardware schemes have been developed for ODT. The most straightforward method to determine the frequency shift involves the use of a short time fast Fourier transform (STFFT). However, the sensitivity of this method is mainly dependent on the FFT time window, which limits axial scanning speed and spatial resolution when measuring slowly moving blood flow in small vessels that requires high velocity sensitivity. However, a phase-resolved technique can decouple the Doppler sensitivity and spatial resolution while maintaining high axial scanning speed.
In ODT, the Doppler frequency shift is proportional to the cosine of the angle between output and input scattering directions of the probe beam and the scatterer's flow direction. When the two directions are perpendicular, the Doppler shift is zero. Because a priori knowledge of the Doppler angle is not available, and conventional intensity OCT imaging provides a low contrast image of microvasculature structure, detecting small vessels with slow flow rates is difficult. However, the Doppler angle can be estimated by combining Doppler shift and Doppler bandwidth measurements. (See for example, Piao D Q, Zhu Q, Quantifying Doppler Angle and Mapping Flow Velocity by a Combination of Doppler-shift and Doppler-bandwidth Measurements in Optical Doppler Tomography, Applied Optics, 2003, 42(25): 5158-5166, and U.S. Pat. No. 5,991,697 describe a method and apparatus for Optical Doppler Tomographic imaging of a fluid flow in a highly scattering medium comprising the steps of scanning a fluid flow sample with an optical source of at least partially coherent radiation through an interferometer, which is incorporated herein by reference).
The ability to locate precisely the microvasculature is important for diagnostics and treatments requiring characterization of blood flow. Recently, several efforts to increase blood flow contrast mechanisms have been reported including protein microspheres incorporating nanoparticles into their shells, plasmon-resonant gold nanoshells, and use of magnetically susceptible micrometer sized particles with an externally applied magnetic field. (See for example, Lee T M, Oldenburg A L, Sitafalwalla S, Marks D L, Luo W, Toublan F J J, Suslick K S, Boppart S A, Engineered microsphere contrast agents for optical coherence tomography, Optics Letters, 2003, 28(17): 1546-1548; Loo C, Lin A, Hirsch L, Lee M H, Barton J, Halas N, West J, Drezek R. Nanoshell-enabled photonics-based imaging and therapy of cancer. Technology in Cancer Research & Treatment, 2004; 3(1): 33-40; and Oldenburg A L, Gunther J R, Boppart S A, Imaging magnetically labeled cells with magnetomotive optical coherence tomography, Optics Letters, 2005, 30(7): 747-749.)
Wang, et al., “Characterization of Fluid Flow Velocity by Optical Doppler Tomography,” Optics Letters, Vol. 20, No. 11, Jun. 1, 1995, describes an Optical Doppler Tomography system and method which uses optical low coherence reflectometry in combination with the Doppler effect to measure axial profiles of fluid flow velocity in a sample. A disadvantage of the Wang system is that it does not provide a method to determine direction of flow within the sample and also does not provide a method for generating a two-dimensional color image of the sample indicating the flow velocity and directions within the image.
The use of an externally applied field to move magnetically susceptible particles in tissue has been termed magneto-motive OCT (MM-OCT). Functional magnetic resonance imaging (fMRI) detects deoxyhemoglobin which is a paramagnetic molecule. However, the paramagnetic susceptibility of human tissue is very low compared to other biocompatible agents such as ferumoxides (nanometer sized iron oxide particles). Therefore, it was believed that, other than differentiating relaxation times (T2) between oxygenated and deoxygenated blood, the magnetic field strength required to produce a retarding force on blood flow was well above that of current imaging fields. (See also, for example, Schenck J F., Physical interactions of static magnetic fields with living tissues, Progress in Biophysics and Molecular Biology 2005, 87 (2-3): 185-204; and Taylor D S, Coryell, C. D., Magnetic susceptibility of iron in hemoglobin. J. Am. Chem. Soc. 1938, 60:1177-1181.)